This invention relates to a model or toy aircraft, in which lift and propulsion of the craft is derived from flapping wings, more or less like a bird. Such a craft is commonly termed an ornithopter.
Traditionally, an ornithopter is powered by means of a rubber drive-band. The arrangement is that a person stores energy in the drive-band by progressively twisting the band. The stored energy is applied to the wings when the person releases the band.
Ornithopters have developed into two categories, being the adult model category and the children""s toy category.
There are ornithopter hobby clubs for adult enthusiasts of flapping flight. The adult ornithopter is characterised as very light, but very fragile, being made of balsa wood, etc.
The children""s toy model is (has to be) much more robust, and consequently the toy version of the ornithopter is generally heavier. Slow-flapping wings would not produce enough lift for the (heavy) toy, and so the designer has to see to it that the wings flap at rather high speed. Even so, the flight characteristics of the traditional toy ornithopter are rather poor. The toy aircraft tend to fly erratically, and tend to stall, not glide, when the power runs out. When the ornithopter is flown indoors, by a child, the associated adults may become irritated by the fast-flapping device in the confined space.
An aim of the invention is to provide a children""s toy ornithopter aircraft that is robust enough to stand up to being handled and operated by children, yet which is light in weight and aerodynamically sound enough that the flap speed can be relatively slow, whereby the aircraft can be flown indoors in a pleasurable manner.
It is an aim of the invention to provide a children""s toy ornithopter that is educational, being open as to its structure, whereby the child user can readily observe and understand the mechanism that leads to powered flight.
A small ornithopter of the children""s-toy type, with which the invention is concerned, is shown in patent publication U.S. Pat. No. 781,104 (1905, Slinn). Also relevant is U.S. Pat. No. 5,163,861 (1992, van Ruymbeke).
In the aircraft depicted by Van Ruymbeke, the airframe of the aircraft comprises a hollow fuselage. The left and right wings are pivoted at wing pivot-points that are structured into the left and right side walls of the fuselage, whereby the wing pivot-points lie each a centimetre or so from the axis of the aircraft, i.e the left and right pivot-points are spaced about two cm apart.
In Van Ruymbeke, the wings have respective inward-extending stubs, which extend almost to the axis of the aircraft. The inner ends of these stubs connect to left and right connecting-rods of the drive mechanism; as the stubs are driven upwards, so the wings pivot about their pivot-points, and so the main, outer, portions of the wings are driven downwards.
The aircraft depicted by Slinn has, by contrast, a solid (i.e not hollow) fuselage. In Slinn, the wings are mounted and driven in a manner that might be noted as opposite to the manner shown in Van Ruymbeke, in that, in Slinn, the wings are pivoted to the (solid) fuselage at the wing""s inner extremity, not at an intermediate point. In Slinn, the left and right crank-arms are provided by the wire frame 13,14, which drives the wings up/down as it rotates.
It may be noted that, in Slinn, the wire components slide over each other in a manner that gives rise to a good deal of rubbing friction, and this design must be regarded as being very poorly engineered.
The drive mechanism of Van Ruymbeke is much more complex, but one benefit is that the forces are applied at more advantageous angles, and there is much less inherent friction associated with converting the energy stored in the coiled rubber band into up/down flapping motions of the two wings.
It may be noted that the pivoting wings of Van Ruymbeke are a different class of lever from the pivoting wings of Slinn. Where the wing pivots at an intermediate point, as in Van Ruymbeke, the force on the connecting-rod is the sum of the force on the wing pivot plus the lift force developed by the wing. Where the wing pivots at its inner end, as in Slinn, the force on the connecting-rod is the difference between those two forces. Therefore, when the wings are pivoted at their inner ends, the forces and frictions associated with the movements of the connecting-rod are reduced.
It is an aim of the invention to achieve a wing-flapping mechanism that is very simple, and yet robust, and is inherently of low friction. The aim is to provide an aircraft which, though robust, flaps better, i.e more slowly, and to achieve this by the arrangement of the components, whereby robustness need not be severely compromised in order to achieve lightness of weight.
It is also an aim of the invention to provide a kit, which can be assembled into an ornithopter aircraft, by a young person, without fasteners, and without the use of tools.